How often should you change a filter in a fish tank?

I'm in the process of cleaning out my fish tank. I bought those tetra time strip filters and the time strip just broke. The stuff inside came out and spilled everywhere. Needless to say, it isn't going to work now. Does anyone know the cycle of one of those time strips or how often you should change the filter in a 10 gallon fish tank? It is just a regular fish tank, nothing special. Thanks!!